An implant is generally understood as any medical device made of one or more materials which is intentionally introduced into the body and is either partially or entirely covered by an epithelial surface. Implants may be divided into temporary and permanent implants in regard to the usage time. Temporary implants remain in the body for a limited time. Permanent implants are intended to remain permanently in the body. Furthermore, implants may be differentiated into prostheses and artificial organs. For purposes of the present disclosure, a prosthesis is a medical device which replaces limbs, organs, or tissue of the body, while an artificial organ is understood as a medical device which partially or entirely replaces the function of a bodily organ. For example, implants such as orthopedic or osteosynthetic implants, cardiac pacemakers and defibrillators, and vascular implants fall under the cited definitions.
An implant material is a nonliving material which is used for an application in medicine and interacts with biological systems. A basic requirement for the use of a material as an implant material, which is in contact with the bodily environment when used as intended, is its bodily compatibility (biocompatibility). For purposes of the present disclosure, biocompatibility is understood as the capability of a material to cause an appropriate tissue reaction in a specific application. This includes an adaptation of the chemical, physical, biological, and morphological surface properties of an implant to the receiving tissue with the object of a clinically desirable interaction. The biocompatibility of the implant material is also a function of the time curve of the reaction of the biosystem in which the implant material is implanted. Thus, relatively short-term irritations and inflammations occur which may result in tissue changes. Toxicity, allergies, or even cancer formation are the moderate to long-term results of a lack of biocompatibility.
Biological systems react differently to foreign bodies as a function of the properties of the material or component. The implant materials may be divided into bioactive, bioinert, and degradable/resorbable materials in accordance with the reaction of the biosystem.
For purposes of the present disclosure, only metallic implant materials are of interest, whose application is in osteosynthesis, joint replacement, dental surgery, and vascular surgery, for example. Biocompatible metals and metal alloys or permanent implants comprise rustproof steels (e.g., 316L), cobalt-based alloys (e.g., CoCrMo cast alloys, CoCrMo forged alloys, CoCrWNi forged alloys, and CoCrNiMo forged alloys), pure titanium and titanium alloys (e.g., cp titanium, TiAl6V4, or TiAl6Nb7), and gold alloys. In the field of biocorrodible implants, the use of magnesium or pure iron as well as biocorrodible base alloys of elements magnesium, iron, zinc, molybdenum, and tungsten are desirable.
A biological reaction to metallic elements is a function of the concentration, action time, and type of the supply. The presence of an implant material frequently results in inflammation reactions, whose triggers may be mechanical irritations, chemical materials, but also metabolic products. The inflammation reaction is typically accompanied by the immigration of neutrophilic granulocytes and monocytes through the vascular walls, the immigration of lymphocyte effector cells with formation of specific antibodies against the inflammation stimulus, the activation of the complementary system with release of complementary factors, which act as mediators, and finally the activation of blood coagulation. An immunological reaction is usually closely connected to the inflammation reaction and may result in sensitization and allergy formation. Known metallic allergens comprise, for example, nickel, chromium, and cobalt, which are also used as alloy components in many surgical implants.
In addition to its biocompatibility, the implant material must, of course, also always fulfill its functional tasks, namely, for example, at least temporarily ensuring the mechanical integrity of the implant and possibly shielding an implant interior in relation to the surrounding material. Because of this, frequently only compromises between all of the requirements to be fulfilled may be implemented on the material in the material selection.
Furthermore, it is known that a higher degree of biocompatibility may be achieved if metallic implant materials are provided with coatings made of especially tissue-compatible materials. These materials are usually of an organic or synthetic-polymer nature and are partially of natural origin. In spite of the progress achieved, there is still a high demand for at least alternative approaches. Thus, for example, the problem frequently arises that the coatings to be applied only adhere inadequately, in particular, in regard to the conditions existing during the implantation.
Furthermore, in manifold medical implants, the most solid and/or secure anchoring possible of the implant on the implantation location is desired. Fundamentally, especially in metallic permanent implants, the problem arises that an implant may not be integrated permanently in the meaning of biological ingrowth into the cell composite. This is usually expressed in the formation of a layer similar to connective tissue around the implant. This layer similar to connective tissue prevents the direct contact of the cells of the surrounding tissue with the implants; the implant is only still anchored by a form fit, but not by biological adhesion, by the cells in this case.
In the case of permanent implants made of titanium and titanium materials, direct colonization of the implant surface with vital cells is frequently observed, but in many cases the layer similar to connective tissue described here also occurs. In addition, an increasing tendency toward loosening and the formation of a layer similar to connective tissue is also frequently observed with progressing implantation time in titanium implants. In rare cases, bioincompatible to cytotoxic reactions also occur in titanium implants at the interface between implant and tissue, especially in the event of longer implantation times.
These reactions are even more significantly pronounced in the case of other implant materials, such as CoCr alloys or implant steels.
Special textures of the implant surface, which make colonization with cells easier and are to encourage the formation of contact points between cell and implant surface, also only inadequately solve the problem. Functional surfaces which are to suppress the formation of a layer similar to connective tissue by special coatings are still predominantly in the research stage; there are not yet reliable findings about the long-term suitability of these layers, which are frequently monomolecular.
If the experimentally available implants up to this point, made of the so-called biologically degradable materials, are also taken into consideration in this observation, biological incompatibilities up to necrosis of the tissue are also found here, especially if an accumulation of the degradation products of such metals is in contact with tissue. Such results have been found, for example, in the evaluation of zinc and zinc-based alloys as implant materials for vascular implants and are to be expected upon the use of degradable implants made of iron, iron alloys, tungsten, and other metals fundamentally degradable in the body, because the body identifies the compounds formed as foreign bodies and a foreign body reaction occurs. Such a reaction typically also results in activation of the immune system connected with a local inflammation. These impairments also result in a slowed healing procedure, connected with a delayed colonization with cells, the formation of layers similar to connective tissue, and a delayed healing procedure.
The surface of orthopedic implants made of the titanium alloy TiAl6V4 may be modified by ion beam implantation using magnesium ions in such a manner that the colonization by human osteoblasts is made easier (Zreiqat et al.; “The effect of surface chemistry modification of titanium alloy on signaling pathways in human osteoblasts”; Biomaterials; 2005; pp. 7579-7586). The ion implantation results in an enrichment of magnesium to a content of approximately 10 atomic-% to a depth of approximately 60 nm of the material. The entry depth of magnesium is a function of the energy of the incident ions, i.e., enrichment of the magnesium occurs in specific surface-proximal layers of the material depending on the energy profile. Conversion of the implant surface, which still contains or comprises titanium dioxide in the event of titanium and titanium alloys, does not occur.